Debugging coreboot in qemu environment - part 2

In previous post coreboot was configured and installed. Here we try to establish good debugging environment for it. To create a good emulated environment to debug, research and learn coreboot few tricks are required. First of all we need to know how to run our emulated enviroment (qemu). What I mean by that ?

• load coreboot image (-bios option),
• freeze CPU at startup (-S),
• get appropriate feedback about virtual machine state (-d in_asm,cpu),
• set up remote gdb server to run qemu step by step (-s).

So finally we get:

qemu -bios src/coreboot/build/coreboot.rom -s -S -d in_asm,cpu -nographic

We don't need graphics so it also could be disable (-nographic). Run above command and prepare debugging environment as described below.

1. Set up gdb:
   1. load bootblock file in gdb:

      file path/to/coreboot/build/bootblock.elf

   2. use objdump to find out at what address .text, .bss and .data sections are:

      objdump -h src/coreboot/build/coreboot_ram|grep -E "text|bss|\.data"

      my output looks like that:

      0 .text         00010810  00100000  00100000  00001000  2**2
      3 .data         000004d8  001174e8  001174e8  000184e8  2**2
      4 .bss          0000080c  001179c0  001179c0  000189c0  2**3

   3. use above addresses to load symbols from coreboot_ram file in gdb:

      add-symbol-file src/coreboot/build/coreboot_ram 0x00100000 -s .data \
      0x001174e8 -s .bss 0x001179c0

2. In another terminal or screen window

   vim /tmp/qemu.log

   (use :e to reload qemu.log file after every instruction), in this file we will get information about all registers of virtual machine
target remote :1234
3. Run next instruction (ni command in gdb) and refresh qemu.log, if you get something like:

   EAX=00000000 EBX=00000000 ECX=00000000 EDX=00000633 
   ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
   EIP=0000fff0 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
   ES =0000 00000000 0000ffff 00009300
   CS =f000 ffff0000 0000ffff 00009b00
   SS =0000 00000000 0000ffff 00009300 
   DS =0000 00000000 0000ffff 00009300
   FS =0000 00000000 0000ffff 00009300
   GS =0000 00000000 0000ffff 00009300
   LDT=0000 00000000 0000ffff 00008200
   TR =0000 00000000 0000ffff 00008b00
   GDT=     00000000 0000ffff
   IDT=     00000000 0000ffff
   CR0=60000010 CR2=00000000 CR3=00000000 CR4=00000000
   DR0=00000000 DR1=00000000 DR2=00000000 DR3=00000000
   DR6=ffff0ff0 DR7=00000400
   

it means that your debugging enviroment was set correctly.